Fix "Remove source branch" button in MR widget during MWPS state

What does this MR do?

• Fix "Remove source branch" button in MR widget during MWPS state

Todo

• Use the actual service in JS tests to avoid method/property mismatches to actual code
• Add rspec test to make sure this works e2e

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #42814 (closed)